SEQUENCES, CYCLES AND HIATUSES IN THE UPPER ALBIAN-CENOMANIAN OF THE IBERIAN RANGES (SPAIN) - A CYCLOSTRATIGRAPHIC APPROACH

The Upper Albian-Cenomanian deposits in the Iberian Ranges are compose d of a complex alternation of continental (sandstones), coastal (lime sandstones and marls) and marine (carbonates) facies. Facies are arran ged mainly in transgressive and deepening-upward sequences and paraseq uences, but deepening-shallowing-upwards sequences and parasequences a lso exist. Depositional sequences and parasequences are bounded by sub aerial erosive surfaces or ferruginous crusts, and by their correlativ e conformities. It is evident from the facies alternation that deposit ion reflects a cyclic process in which is superimposed several orders of cycles of relative sea-level rise and fall. The cycles are preserve d as 3rd-order depositional sequences and 4th- and 5th-order discontin uity-bounded parasequences, the latter being the basic blocks of the f ield record. A cyclostratigraphic model for sedimentation shows that e ach of the 3rd-order cycles is composed of five 4th-order cycles. A 4t h-order cycle is also composed of five 5th-order cycles. This 1:5:5 ra tio reflects the maximum number of parasequences observed in the most complete and marine sections. The ratio remains unchanged throughout t he basin, except by onlap caused by relative sea-level rises or trunca tions caused by relative sea-level falls. A detailed correlation of se ctions separated by more than 200 km shows that thickness differences are caused mainly by parasequence disappearance, not parasequence thin ning. Cycles are mainly eustatic in origin, controlled by accommodatio n space. Increased accommodation generates thick parasequences of wide r regional extension; they contain the more marine facies in any secti on. Only some sequences with a restricted regional distribution are re corded if the accommodation is low. The average duration of 3rd-order cycles was 1.95 Ma (ranging from 1.33 to 2.66 Ma), an average 4th-orde r cycle was 390 ka (266-533 ka) and a 5th-order cycle was 78 ka (53-10 6 ka).